PURPOSE: To prevent the resonance of a branch phone by stopping the input of a ringer detecting signal to an input port by means of a second transistor when the terminal voltage of a capacitor exceeds a setting value. ;CONSTITUTION: When a ringer signal for calling is inputted from a telephone line to a pull-in line 1, only the ringer signal where a DC portion is removed is inputted to the light emitting element 5 of a photocoupler 4 with a register 3 and the element 5 is turned on/off every two seconds. Therefore, the phototransistor (TR) 6 is turned on/off by the timing, a capacitor 9 is charged at the time of turning-off and also a second TR 16 is turned-on so as to keep the input port 15 of IC for detecting ringer 14 L. When the TR 6 is turned on, TR 16 is turned off and also the capacitor 9 is discharged by a discharge register 11 with TR 6. The terminal voltage of the capacitor 9 is successively reduced, TR 12 is turned on when the voltage becomes equal to below a set level, the voltage of a power source line 7 is inputted to the input port 15 with the register 13 and the ringer signal is detected.;COPYRIGHT: (C)1993,JPO&Japio
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